1. Field of the Invention
The invention relates to an electromagnetic deflection unit for a television picture tube whose neck accommodates an electron gun system for directing at least one electron beam to a display screen, the electro-magnetic deflection unit being arranged around the envelope of the display tube and comprising a first deflection coil and a second deflection coil which is located more closely to the envelope of the display tube and coaxially with respect to the first deflection coil, each coil having a front end facing the display screen and a rear end, the second deflection coil being of the saddle type.
2. Description of the Prior Art
In monochrome display tubes the electron gun system is adapted to produce one electron beam. in colour display tubes the electron gun system is adapted to produce three electron beams.
In monochrome display tubes for, for example, data display uses and for TV projection tubes, the aim is to use a deflection unit having deflection coils which yield such a field distribution that the spot quality in all areas of the display screen, also in the corners, is as perfect as possible.
For some time colour display tubes have been used in which three spatially separated electron guns are located in one line. A display tube of this type is known as an in-line colour display tube. In the in-line colour display tube the aim is to use a deflection unit having deflection coils yielding such an inhomogeneous field distribution that the beams of the electron guns coincide across the entire screen when they are deflected. To this end particularly the horizontal deflection field (to be generated by the second deflection coil) should be barrel shaped toward the gun side of the deflection yoke and should be pincushion-shaped towards the sreen side thereof, and conversely the vertical deflection field (to be generated by the first deflection coil) should be pincushion-shaped toward the gun side of the deflection yoke and barrel shaped towards the screen side thereof.
The extent of the pincushion and barrel shaping is such that during deflection the convergence errors of the electron beams emitted by the electron guns are corrected so that pictures having satisfactory convergence properties can be produced on the screen of the display tube. Display tube--deflection yoke combinations of this type are referred to as self convergent.
If the spot quality (in minochrome tubes) or the convergence (in colour tubes) is ensured in the manner described above (in practice the deflection coils must often be combined with auxiliary means for enhancing the pincushion and/or barrel shape of the deflection fields, which auxiliary means may be, for example, plates of a soft magnetic metallic material placed in the deflection fields) it is still often found that a disturbing (pincushion-shaped) raster distortion (north-south, and east-west raster distortion) occurs on the edges of the display screen, which distortions must be corrected.
Until recently such raster distortion correction had been sought by using permanent magnets (referred to as raster magnets) or by providing a wire distribution in the line and/or the field deflection coil such that a strong 6-pole magnetic field component is produced upon energizing. However, now that the display screen has become increasingly flatter in the latest types of display tubes, the known correction methods fall short.